Actually, it is generally possible to transport between different releases -- just not usually recommended. In newer Basis releases there is a check that prevents the import by default, but the transport administrator can check a flag during import to bypass that check. This is all config and/or custom stuff, right? That usually isn't so much of a problem. It's mods to standard objects that are the real danger (and definitely not recommended).
I always tell my folks "we can't do (normally) it" so as to push them to more rigorous standards during support pack or upgrade projects, but there are always emergencies, and it can definitely be done.